Attacking planets with .99c moving objects is one of those things that the writers ignore because it would break the story.
Like the implications of an awful lot of their technology. Matter-energy conversion, teleportation, gravity control ....
Bad Thoughts wrote:
If you could create enough energy to push an asteroid near the speed of light, why bother with the asteroid. Just send the energy.
Sounds like someone has been reading James Hogan.
And you're right.